Adding GFCI but leaving existing circuit

Please see the attached sketch before looking at my explanation.
Basically I want to remove a receptacle and replace it with a GFCI on a new dedicated circuit.
The problem is the power to the existing receptacle also feeds a light in my bathroom. If I replace the existing box with a larger 4"x4"x2" box, is it OK by code, to 1.) use that box as a junction box to reconnect the existing circuit(power to light) and 2.) connect the GFCI on the new dedicated circuit?